This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-153508, filed May 24, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a half-transmissive liquid crystal display element.
Conventionally, for example, a reflective liquid crystal display element is designed to utilize external light for display. In some use environments, therefore, illumination is insufficient so short that its display screen is dark. It thus cannot be used at all in a dark place.
Meanwhile, developments have been made for a half-transmissive liquid crystal display element which uses a half-transmissive reflection plate (half-mirror) as a reflection plate for reflecting external light and a back light is provided on the back surface of this half-transmissive reflection plate, so that the element can be used as a reflective liquid crystal display element in a bright environment and as a transmissive liquid crystal display element in a dark environment. However, the half-transmissive reflection plate utilizes incident light at an efficiency of 50% at most, and therefore, the brightness of the display screen is far lower than a transmissive liquid crystal display element or a reflective liquid crystal display element.
In recent years, in response to the problems described above, a half-transmissive liquid crystal display element is studied such that a pinhole is formed in the reflective layer for each pixel and a micro-lens is provided for each pixel. In this liquid crystal display element, the efficiency of use of light is raised by using external light reflected at the area of a reflection plate excluding the pinholes, as a light source when using external light, and by using light which passes through the pinholes and is collected by the micro-lens.
In this kind of liquid crystal display element, however, the brightness of its display screen is lowered only by a margin corresponding to the pinhole when using external light. As a result, the frequency of use as a transmissive liquid crystal display element using the back light is increased so that the power consumption increases. In addition, the reflection plate comprising pinholes has a complicated structure and must therefore be constructed as a reflection plate externally attached to a liquid crystal panel. As a result, a parallax is caused which considerably deteriorates the display performance.
Meanwhile, discussion has also been made on a display element of a so-called front-light type in which a light guide plate is provided on the observation surface side of a reflective liquid crystal display element and a linear light source is provided on the side surface of this light guide plate. However, in this display element, a surface reflection at the front surface of the display element is large so that the image quality such as contrast is greatly lowered.
The present invention has been made in view of the circumstances described above and its object is to provide a half-transmissive liquid crystal display element in which the light use efficiency is greatly improved so that excellent display can be realized over a wide view angle range.
To achieve the above object, a half-transmissive liquid crystal display element according to the present invention comprises: a polarization plate having a polarization axis, for transmitting linearly polarized light along the polarization axis;
a light modulation layer arranged behind the polarization plate, for modulating incident light having first and second circular polarization components, in correspondence with an applied voltage;
a selectively reflective layer arranged behind the light modulation layer, for reflecting the first circular polarization component of incident light; and
a back light source arranged behind the selectively reflective layer, for emitting light having intensity peaks at a plurality of predetermined wavelengths, respectively, toward the selectively reflective layer, wherein
the selectively reflective layer has first reflection factors to the first polarization components of incident light falling within first small regions of visible light, including the plurality of predetermined wavelengths, and has second reflection factors to the first polarization components of incident light falling within second small regions of visible light, not including the plurality of predetermined wavelengths, the first reflection factors being set to be smaller than the second reflection factors.
In the liquid crystal display element according to the present invention, the first reflection factors are set to be 30 to 70% of the second reflection factors.
Further, in the liquid crystal display element according to the present invention, in at least one group of the first and second reflection factors, the reflection factors are not constant.
According to the liquid crystal display element constructed as described above, the selectively reflective layer transmits light from the back light source to use it mainly for display, within a wavelength range in which the intensity of light from the back light source is high. Within a wavelength range in which the intensity of the light of the back light source is low, the selectively reflective layer reflects light, which is taken in from outside into the element, to use the light mainly for display. Therefore, the light of the back light source and the external light can be efficiently used.
Also, in the liquid crystal display element according of the present invention, at least one of the first small regions has a center wavelength longer than a corresponding one of the predetermined wavelengths.
Also, in the liquid crystal display element according to the present invention, the center wavelength of the at least one of the first small regions is longer by 0 to 40 nm than the corresponding one of the predetermined wavelengths. Further, in the liquid crystal display element according to the present invention, each of the first small regions has a band width of 30 nm to 80 nm.
In at least one of the first small regions, a minimum value of the reflection factor to the first circular polarization component is equal to or longer than the corresponding one of the predetermined wavelengths.
According to the liquid crystal display element constructed as described above, sufficient use efficiency of light can be maintained if a selected reflection wavelength shifts to the side of a shorter wavelength when the liquid crystal display element is observed obliquely. It is thus possible to attain display characteristics in which visual changes in colors and brightness are reduced.
Another liquid crystal display element according to the present invention comprises: a polarization plate having a polarization axis, for transmitting linearly polarized light along the polarization axis;
a light modulation layer arranged behind the polarization plate, for modulating incident light having first and second circular polarization components, in correspondence with an applied voltage;
a selectively reflective layer arranged behind the light modulation layer, for reflecting the first circular polarization component of incident light; and
a back light source arranged behind the selectively reflective layer, for emitting light having intensity peaks at a plurality of predetermined wavelengths, respectively, toward the selectively reflective layer; and
a color filter layer provided in front of the selectively reflective layer, wherein
the selectively reflective layer has first reflection factors to the first polarization components of incident light falling within first small regions of visible light, including the plurality of predetermined wavelengths, and has second reflection factors to the first polarization components of incident light falling within second small regions of visible light, not including the plurality of predetermined wavelengths, the first reflection factors being set to be smaller than the second reflection factors, and
the color filter layer has spectral transmission factors to incident light, the spectral transmission factor to incident light falling within at least one of the first small regions being set to be smaller than the spectral transmission factor to incident light falling within the second small regions.
According to the liquid crystal display element constructed as described above, color densities of the color filter layer can be individually set for a wavelength range which is used mainly for reflection and for a wavelength range which is used mainly for transmission, respectively. Display with a wider color reproduction range can thus be achieved.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.